You must use a steel with a Brunnel (sp?) Hardness of at least 400. Look for AR400 as it is much cheaper than T1, and is quite adequte. Chickens and pigs should be 1/2", but the turkeys and rams should be 3/8"- especially the rams. If you use heavier guage, you won't be able to knock them down.
Our first targets were made from AR250, and that was too soft. Don't go much higher than 400, as it gets too brittle. A 4x10 sheet of T1 goes for about $900, while a 4x8 sheet of AR400 goes for about $385 in these parts. Have plenty of help on hand to load and unload- the sheets are extremely heavy. Blamethewind.